Your boiler is still detecting a flame even though it has tried to turn the gas supply off, which is a safety conflict.

Flame detected after gas has been shut off

What causes this fault?

This fault occurs when the boiler's internal sensors continue to detect heat or a flame after the heating cycle has finished and the gas should be off. The most common reason is that the gas valve has become stuck open or 'leaky,' or the sensing electrode is sending a false signal back to the main control board. Because the system cannot confirm the fire is actually out, it enters a high-security shutdown to prevent overheating.